The invention relates generally to a high-power rotary electrodynamic machine, and more particularly to a high-power generator or motor that operates at a relatively high rotational speed and has dual rotors.
A generator may be used to convert mechanical energy from a prime mover into electrical energy. A motor performs the opposite function. For simplicity, the discussion hereinbelow is limited primarily to generators.
Generators typically use a rotating member known as a rotor mounted within a stationary member known as a stator. The rotor is rotatably driven by a prime mover. In an aircraft, a generator may be driven by means of a main or auxiliary engine by means of a gearbox, a constant speed drive (CSD) transmission, an engine starter, etc. The electrical energy produced by the generator illuminates the cabin, powers avionics, heats food, etc. Electrical power requirements typically are greater for newer aircraft as compared with their predecessors because more electrical devices and loads are used; in particular, flight control surfaces are increasingly being actuated by electric power rather than hydraulics.
In general, it is advantageous to design a generator without brushes or slip rings for conducting electrical current to or from the rotor. Brushes and slip rings wear and thus reduce the reliability of the generator. For relatively small power needs, a generator without brushes or slip rings (a xe2x80x9cbrushlessxe2x80x9d generator or alternator) may be designed by placing a conductor such as a winding in a stator and providing one or more permanent magnets within a rotor. When the rotor is driven, the resulting rotating magnetic field induces a current in the stator conductor. The conductor may then deliver the induced current to an electrical load.
When the need for electrical power delivered by the generator is relatively large, however, a rotor winding is generally used instead of a permanent magnet. The rotor winding, or main field winding, becomes an electromagnet when the winding is connected to a current source. The rotor winding may be turned off if a short circuit occurs. The winding produces a rotating magnetic field of sufficient intensity to generate the relatively large power. This magnetic field may be regulated by regulating the current to the main field winding. The main field winding must be relatively rigid, compact, and balanced so that it may be rotated at high speed without undue deformation or vibration. Some means for supplying the electrical current to the main field winding must be provided, however, preferably without resorting to slip rings or brushes.
It is known to supply the current to the main field winding without using brushes or slip rings by using magnetic induction. Magnetic induction is best understood by reference to a typical prior art brushless alternator. A prior art brushless alternator uses three distinct generators: a main generator, an exciter generator, and a permanent magnet generator (PMG). Each of the generators comprises a rotating member integral to a common rotor of the brushless alternator and a stationary member integral to a common stator assembly of the brushless alternator. The common rotor is typically rotatably supported by two bearings.
The rotating member of the PMG, which includes one or more permanent magnets, creates a rotating magnetic field when the rotor is driven by the prime mover. The rotating magnetic field induces an alternating current (AC) in a stationary PMG armature winding located within the stator of the brushless alternator. This induced AC in the PMG armature winding is typically rectified and voltage regulated in a stationary rectifier connected to the stator to supply a direct current (DC) to a stationary field winding of the exciter generator, also located within the stator. This field winding uses the DC to produce a stationary magnetic field. Within that field, an exciter armature winding integral to the rotor is rotated to generate a higher level of current than the PMG current output, typically in the form of a three-phase AC. To generate the desired magnetic field in the rotating main field winding, DC, not AC, must be used. Because the output of the exciter armature winding is AC, a rotating rectifier assembly located within the rotor is typically used to rectify this AC to DC. This DC is connected to the main field winding in the rotor. Finally, this main field winding generates a rotating magnetic field that induces AC into the main generator stator and then to a load. The main field winding generally comprises a plurality of coils of wire wound around a magnetic core. This arrangement is commonly referred to as xe2x80x9cpoles.xe2x80x9d
When relatively lower electrical output power is required from a generator, at typical aircraft power frequencies (e.g., approximately 350 to 800 Hz), wire-wound rotors may operate below the first xe2x80x9ccritical speedxe2x80x9d, i.e., at xe2x80x9csubcriticalxe2x80x9d speed. By definition, the first xe2x80x9ccritical speedxe2x80x9d is the speed at which the rotor is in its first dynamic resonance mode. At this speed, the rotor bends and displaces radially. At or above the critical speed, bearing loads increase and rotor deflection is magnified; a risk exists that the rotor will rub against the stator. Subcritical operation also precludes the need for additional torque in the prime mover to force the rotor to pass quickly through the critical speed. In addition, subcritical operation reduces harmful vibration for the generator and for the surrounding aircraft structures. Thus, a generator rotor preferably is operated at subcritical speed.
When relatively greater generator output power is required (because more electrical devices and loads are used in newer aircraft as compared with their predecessors), increasing the rotor weight and the xe2x80x9cbearing span,xe2x80x9d i.e., the distance between bearings supporting the rotor, typically results in a design using relatively slower rotor speed and an increased number of poles for a given output frequency range. The increased number of poles and heavier rotor inherently cause the generator weight to increase. However, the aerospace industry is always attempting to reduce the size and weight of aerospace components. One way to reduce the size and weight of generators while achieving a relatively high electrical output is to design for comparatively high rotor speeds. Modern aircraft generators may operate between approximately 7,000 and 40,000 rpm.
In typical prior art brushless alternators, two bearings support a one-piece rotor that includes the three separate generators mentioned hereinabove. The bearings are typically disposed at each end of the rotor. In general, the larger the required electrical output, the larger the generator and its electromagnetic parts. As the requirement for output power continues to increase in new aircraft, the rotor weight, the bearing span, the bearing rotational speed, the rotor centrifugal forces, and the support stiffness may not permit safe and functional alternator operation.
Prior art attempts to build a generator to operate at high power within weight and size requirements demanded by modern aircraft have failed. The present invention is specifically directed to overcoming the above-mentioned problems.
Accordingly, an object of this invention is to enable high-speed operation of a high-power rotary electrodynamic machine.
An additional object is to provide an improved generator that reduces size and weight while delivering relatively high output power.
Another object is to provide a generator that, compared with those of the prior art, is relatively straightforward to manufacture and repair.
Yet another object is to provide a high-speed, high-power rotary electrodynamic machine that operates below the critical speed of its rotor assembly.
According to the invention, a rotary electrodynamic machine comprises a stator having a plurality of windings; a first rotor being mounted within the stator for rotation about an axis, the first rotor having a magnetic device formed integral therewith, and being disposed in proximity to one of the plurality of stator windings, and a second rotor being mounted within the stator for rotation about an axis, the axis of rotation of the second rotor being coaxial with the axis of rotation of the first rotor, the second rotor having junction and pivot ends, the second rotor being connected to the first rotor at the junction end, the second rotor having a magnetic device formed integral therewith and being disposed in proximity to a second one of the plurality of stator windings.
In further accord with the present invention, the second rotor further includes one or more permanent magnets formed integral therewith and being disposed in proximity to a third one of the plurality of stator windings.
In still further accord with the invention, an external power source provides AC to a second one of the plurality of stator windings.
In yet further accord with the present invention, the junction of the first and second rotors includes means for providing electrical connections between the magnetic device of the second rotor and the magnetic device of the first rotor.
In still further accord with the invention, the rotary electrodynamic machine includes first, second and third bearings, wherein the first rotor is supported at a first end by the first bearing and is supported at a second end by the second bearing, and the second rotor is coupled at the junction end to the first rotor and is supported at the pivot end by the third bearing.
In further accord with the invention, the machine includes means for axially clamping the magnetic devices of the first and second rotors.
The invention has several benefits: it produces relatively large electrical output power while being relatively light and small. The invention operates at subcritical speed. It thereby avoids operation at, or transition though, resonance frequencies of the rotor, thus reducing vibration.
The above and other objects, features, and advantages of this invention will become apparent when the following description is read in conjunction with the accompanying drawings.